The manufacture of paper from recycled paper waste, especially post-consumer waste, has become a major industry. Recycling efforts throughout the United States have made large quantities of recycled paper available. This, combined with the high cost of virgin wood fibers, has raised the economic importance of paper manufactured from recycled fibers.
The value of the paper manufactured from recycled fiber, like the value of paper manufactured from virgin fiber, is dependent on the mechanical properties of the paper and the appearance of the paper. Appearance is chiefly measured in terms of brightness, which for recycled paper means a lack of ink particles and outer dark material endemic to recycled fiber sources.
The principal use of paper is for printing and the vast majority of all printed inks are based on carbon particles. Thus, a pulp manufactured from recycled paper is typically contaminated with a substantial amount of particulate carbon. The carbon from printers' ink is chemically detached from the fiber and must be mechanically separated from the fiber if a high quality sheet is to be manufactured from the recycled stock.
There are three basic ways of cleaning recycled stock: washing the stock, using floatation separation techniques, and using hydrocyclones. Generally all three techniques are used.
Floatation separation techniques can be particularly advantageous, especially the techniques such as those disclosed in U.S. Pat. No. 5,273,624 which is incorporated herein by reference. The U.S. Pat. No. 5,273,624 patent shows how a sealed floatation chamber allows the use of vacuum or pressure within the chamber to simplify the collection and removal of the foam containing the carbon particles.
In general the floatation technique (long utilized in the mining industry) utilizes surfactants and various chemical additives to produce a stable foam or froth. The surfactants preferentially attach to the material to be separated and to the surface of a bubble of air, thus attaching the carbon particle to air bubbles introduced into the stock. The air rises through the pulp stock and is removed along with the carbon particles. Unlike the mining industry, where the material floated to a surface is the desired product, the floatation material in recycling paper is a waste product. Thus the controlling goal is complete removal of carbon from the paper stock and the loss of some fiber with the carbon contamination is acceptable and even desirable.
Although the systems disclosed in the U.S. Pat. No. 5,273,624 patent are a significant improvement on prior systems there is considerable economic incentive and need for systems which are more compact and efficient. Sources of recycled fiber are lower cost than virgin fiber, yet recycled fiber typically requires more processing to improve the quality of the paper manufactured. Thus, any improvement in separation efficiency by reducing the number of cleaning steps makes an important contribution to the economic viability of recycling paper fibers.